Chakotay's grandfather suffered from hallucinations caused by an inherited condition and refused treatment, and appears in The Fight as the figure embodying Chakotay's fear of losing his own mind.
Chakotay's grandfather is seen in the fifth-season Voyager episode The Fight, played by Ned Romero. He carried a hereditary trait that produces a kind of sensory disorder, causing him to see and hear things that others could not.
As a boy Chakotay watched the old man wander off after his visions, and remembers him declining the medicine that would have suppressed them because he valued what he experienced. Chakotay carries the same gene, kept dormant before his birth, and dreads what it could do.
When aliens in chaotic space reactivate the trait in order to speak to him, his hallucinations include his grandfather in the woods of his childhood, urging him to listen. Accepting that example allows Chakotay to complete the contact.